MOCVD method and apparatus

ABSTRACT

In an MOCVD reactor, gases are channeled around the periphery of a baffle plate (15) so as to flow radially inwardly along a slotted injection plate (16). The slots (22) in the injection plate extend radially and are of non-uniform width so as to compensate for a non-uniform rate of deposition. The resultant flow over a rotating heated substrate (17) gives a more uniform deposit of epitaxially grown material.

TECHNICAL FIELD

This invention relates to methods and apparatus for making semiconductordevices, and, more particularly, to methods and apparatus for makingepitaxial layers by metalorganic chemical vapor deposition (MOCVD).

BACKGROUND OF THE INVENTION

One of the most significant developments in semiconductor technology inrecent years has been the increased use of III-V materials such asgallium arsenide and indium phosphide, and their ternary and quaternaryalloys such as indiumgallium-arsenide-phosphide, as the active materialof semiconductor devices. The band gap characteristics of such materialstypically make them candidates for optoelectronic and photonicapplications such as lasers, light emitting diodes and photodetectors.For integrated circuit use, their high electron mobility often makesthem preferable to the more commonly used semiconductor, silicon.Fabrication of such devices often requires epitaxial growth of one ormore layers on a single-crystal substrate (epitaxial growth refers to amethod of depositing a material on a substrate such that the crystalstructure of the deposited material effectively constitutes an extensionof the crystal structure of the substrate).

The three broad classes of methods for depositing by epitaxial growthare liquid phase epitaxy, vapor phase epitaxy and molecular beam epitaxywhich, respectively, involves deposition from a liquid source, a vaporsource and a molecular beam. A particularly promising form of vaporphase epitaxy is a method for depositing from a gas including ametalorganic compound; this process, known as metalorganic chemicalvapor deposition (MOCVD), is described in a number of scientificpublications including, "Metalorganic Chemical Vapor Deposition of III-VSemiconductor," M. J. Ludowise, Journal of Applied Physics, Vol. 58, No.8, Oct. 15, 1985, pp. R31-R55, and the paper, "Metalorganic ChemicalVapor Deposition," P. Daniel Dapkus, American Review of MaterialSciences, Annual Reviews, Inc., 1982, pp. 243-268. MOCVD processes makeuse of a reactor in which a heated substrate is exposed to a gaseousmetalorganic compound containing one element of the epitaxial layer tobe grown and a gaseous second compound containing another element of thedesired epitaxial material. For example, to grow the III-V materialgallium arsenide, one may use the metalorganic gas triethylgallium [(C₂H₅)₃ Ga] as the gallium source and arsine (AsH₃) as the source of thegroup V component, arsenic. The gas mixture is typically injectedaxially at the top of a vertically extending reactor in which thesubstrate is mounted on a susceptor that is heated by a radio-frequencycoil. The gases are exhausted from a tube at the end of the reactoropposite the input end.

While MOCVD offers many recognized advantages over other forms ofepitaxy, several problems remain in the formation of high qualitydevices. Chief among these is the problem of obtaining good uniformityof deposition along the upper surface of the substrate. Since the properoperation of devices such as semiconductor lasers requires severaldifferent epitaxial layers, each only a few microns thick, it can beappreciated that significant deviations of thickness uniformity mayresult in serious differences in the operation of such lasers. Moreover,use of such devices in systems requires a great deal of reproducibilityin their production which cannot be achieved if a sufficient uniformityof deposited layer thicknesses is not obtained.

SUMMARY OF THE INVENTION

The invention is an improvement of a reactor of the type in which MOCVDgases are directed into one end of the reactor containing the heatedsubstrate and exhausted from the other end. The gases are channeled andguided so as to flow through a slot or slots in an injection plate thatis arranged generally parallel to the substrate. The slots have anon-uniform width which allows a non-uniform injection of gases tocompensate for a non-uniform deposition rate. The substrate is rotatedduring the deposition.

Both the substrate and the injection plate are typically horizontallyarranged along the central axis of the reactor. The slots in theinjection plate are preferably arranged radially with respect to thecenter of the injection plate with each slot being narrower at the endnearest the center than at the end nearest the periphery of theinjection plate. A baffle plate is preferably located above and parallelto the injection plate and opposite the substrate. The input gases aredirected against the center of the baffle plate and are guided aroundthe periphery of the baffle plate so that they thereafter flow in aradially inward direction along the surface of the injection plateopposite the substrate.

The embodiments that have been found to be preferred use injectionplates having either two or four radially extending slots. These slotsmay be made in the injection plate by making the plate of quartz,mounting it on an X-Y table, and using known computer-controlledapparatus to drive the injection plate with respect to a laser beam thatcuts a desired pattern in the injection plate corresponding to the slotperiphery. After cutting the pattern representative of a single slot,the interior of the cut portion simply drops from the injection plate.The process is repeated for each of the other slots.

As will be explained later, epitaxial layers made by metalorganicchemical vapor deposition (MOCVD) in accordance with applicants' methodand apparatus have a greater thickness uniformity than can be obtainedby use of comparable apparatus of the prior art. These and otherobjects, features and benefits of the invention will be understood froma consideration of the following detailed description taken inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic sectional view of an MOCVD reactor in accordancewith one embodiment of the invention;

FIG. 2 is a bottom view of the injector plate 16 of the reactor of FIG.1, in accordance with one embodiment of the invention;

FIG. 3 is a schematic view of an injection plate in accordance withanother embodiment of the invention;

FIG. 4 is a schematic view of part of the reactor of FIG. 1;

FIG. 5 is a view of a slot that may be used in the injector plate ofFIGS. 2 or 3;

FIGS. 6 through 11 are graphs of thickness versus radial distance ofvarious epitaxial layers made in accordance with various embodiments ofthe invention;

FIG. 12 is a view of a method for making injection plate slots inaccordance with an illustrative embodiment of the invention.

DETAILED DESCRIPTION

It is to be understood that, while an effort has been made to show therelative sizes of the elements in the drawing, the drawing is notintended necessarily to show the various elements to scale. Also, thefollowing description is made largely for the purpose of describing thenature of the invention and how it is to be used. A working reproductionof the embodiments described presupposes a knowledge of the prior art,as typified by the references cited above.

Referring now to FIG. 1, there is shown an MOCVD reactor having anenclosure 10 and a central axis 11. A gas input manifold 12 comprises agas inlet tube 13, a gas guiding member 14, a baffle plate 15 and aninjector plate 16. A single-crystal semiconductor substrate 17 ismounted on a susceptor 18, which in turn is supported by a supportmember 19. The inlet tube 13 transmits input gases from a source 20 suchas to flow around baffle plate 15 and through slots in injector plate 16such that the gas can impinge on the substrate 17. FIG. 2, which is abottom view of injector plate 16, illustrates the configuration of slots22. Baffle plate 15 is disk-shaped, is solid, and is separated frommembers 14 and 16 by spacer members 23.

The substrate 17 is heated by susceptor 18, which in turn is heated by aradio-frequency coil 24 surrounding the enclosure 10. The substrate 17is a single crystal of a compound semiconductor material such as a III-Vmaterial or a II-VI material. The substrate may illustratively bemonocrystalline n-doped indium phosphide (InP), a III-V material. Thepurpose of the reactor is to cause components of the input gases todeposit on the upper surface of the substrate 17 such as to form anepitaxial layer on the substrate. The gases include a metalorganic gascontaining a metal element of the epitaxial layer to be grown, and agaseous second compound containing another element of the desiredepitaxial material. With the wafer properly heated, these componentsreact near the surface of the wafer to form an epitaxial layer inaccordance with known MOCVD principles. The remaining part of the gas isremoved from the enclosure 10 through an outlet tube 26.

During deposition, the susceptor and wafer are driven to rotate by amotor 27 which equalizes to some extent the thickness of deposition. Alower baffle 28 prevents the asymmetric location of the outlet tube 26from creating asymmetries in the gas flow over wafer 17. With asubstrate of indium phosphide, the metalorganic compound of the inputgas may be trimethylindium [CH₃)₃ In], which is a source of indium, andthe second compound may be phosphine (PH₃), which is a source ofphosphorous. These components react to form an epitaxial layer of indiumphosphide over the indium phosphide substrate 17; that is, the layerconstitutes a crystal extension of the crystal structure of thesubstrate.

In accordance with one feature of the invention, the thicknessuniformity of the deposited epitaxial layer is greatly improved by themethod we use for injecting the input gases into the region abovesubstrate 17. The gas may be injected toward the wafer through fourslots 22 in the injection plate 16 as shown in FIG. 2, or through onlytwo slots 22A as illustrated in FIG. 3. In either case, the slots areradially extending and of varying width; in the FIGS. 2 and 3embodiments, each slot is narrower at the end nearest the center of theinjection plate than at the end nearest the periphery of the injectionplate 16. The purpose of these embodiments is to combat a tendency,common to vertical flow MOCVD reactors of the prior art, for the gasprecursors to deposit more thickly on the center of the substrate 17than at the substrate periphery.

Referring to FIG. 4, with either embodiment, the input gases follow apath 29 through the input manifold 12 as shown so as to be injectedthrough slots in the injection plate 16 toward the substrate 17. Thegases are initially projected against the center of the baffle plate 15,they are caused to flow around the periphery of the baffle plate, andthereafter flow radially inwardly along the upper surface of theinjection plate 16. As they flow radially inwardly, they are injectedthrough slots 22 toward the substrate. Many reactor designs were triedin an effort to combat a tendency in reactors of the prior art for thelayer to deposit more thickly toward the center of the wafer, but onlythe designs shown in FIGS. 1-4, as will be explained more fully later,consistently gave thickness uniformities of ±10 percent deviation overninety percent of the area of the substrate. Another advantage of usingthe baffle plate is that it induces a more laminar flow and a highervelocity to the gas which reduces spurious deposition on the innersurfaces of manifold 12. A circle connecting the outer edges of theslots should have a larger diameter than a circle connecting the outeredges of the substrate, both of which are centered on central axis 11.

Referring to FIG. 4, typical dimensions for the reactor apparatus may beas follows: the distance a between the substrate 17 and the injectorplate 16 may be one-half to four inches; the distance b between theinjector plate 16 and the baffle plate 15 may be 0.05 to 0.2 inch; thedistance c between the baffle plate 15 and the horizontal portion of thegas guiding means 14 may be 0.05 to 0.2 inch; the inner diameter d ofthe inlet tube may be 0.6 inch; the diameter of the baffle plate 15 maybe 4.7 inches, the diameter of the injector plate 16 may be 4.9 inches.Referring to FIG. 5, the length of each of the slots 22 may be 1.19inch, the distance between the far end of the slot and the center of theinjector plate may be 1.25 inch, and the width A of the narrow end ofthe slot and the width B of the wide end of the slot may vary as will bediscussed more fully later.

In demonstrations of the invention that were made, the input gasesincluded hydrogen dilution flow rates of between 5 to 14 standard litersper minute (slm), trimethylindium at 40° C. was transported to thereactor by helium at 0.2 slm, and 10 percent of PH₃ in helium at 0.3slm. Iron and silicon dopants were also used, including small amounts of(C₅ H₅)₂ Fe for the iron, and a gas mixture of fifty parts per millionof SiH₄ in helium for the silicon. Successful deposits were made withthe dimension a of FIG. 4 being varied between 2.5 inch and 3.5 inches.The substrate 17 was comprised of four indium phosphide coupons (waferportions), each having defined in it approximately two thousand indiumphosphide lasers. The substrate was centrally located on the susceptorand had dimensions of 2.0 inch by 1.6 inch. The total gas flow waseleven to sixteen slm. The substrate 17 was rotated at sixty rotationsper minute.

A number of different designs for the injector plate 16 were made witheither two or four slots, as shown in FIGS. 2 and 3 and with variousslot dimensions, A and B, as shown in FIG. 5. Table 1 summarizes thedifferent injector plate designs with different slot dimensions.

                  TABLE 1                                                         ______________________________________                                               Number and    Slot Dimension                                                  Orientation   (inches)                                                 Design # of Slots        A      B                                             ______________________________________                                         7       4 at 90° 0.011  0.018                                          8       4 at 90° 0.009  0.018                                          9       4 at 90° 0.014  0.018                                         10       4 at 90° 0.012  0.018                                         11        2 at 180°                                                                             0.023  0.032                                         12        2 at 180°                                                                             0.023  0.033                                         ______________________________________                                    

FIGS. 6-11 show the thickness variations for various films grownepitaxially with the designs 7-12 of Table 1. For example, in Table 1,Design Number 7 uses four slots arranged at 90°, as shown in FIG. 2,with the narrow and wide dimensions A and B of FIG. 5 being 0.011 and0.018 inches, respectively. With this design, four runs were made underdifferent conditions resulting in the four curves shown in FIG. 6. Mostof these show thickness variations of less than plus or minus tenpercent over ninety percent of the area of the substrate. Some of thedesigns clearly lead to more uniform layer thicknesses than others andno effort has been made to correct such data even though certain runsshow greater non-uniformity. Nevertheless, it can be seen that, withoutundue experiment, the invention may be used to arrive at designs usingboth two and four slots having a high degree of thickness uniformity andreproducibility. Table 2 shows some of the variations in uniformity as afunction of hydrogen flow for Design Numbers 11 and 12. The term "fused"indicates that the inlet tube 13 was permanently fused to enclosure 10.

                                      TABLE 2                                     __________________________________________________________________________    Uniformity of Layers                                                                             Average                                                               Hydrogen Flow                                                                         Growth Rate                                                                          Uniformity                                          Design #                                                                             Run #                                                                             (slm)   (μm/hr)                                                                           100% Area                                                                           90% Area                                      __________________________________________________________________________    standard jar                                                                         2603                                                                              12      5.1    13%    11%                                          11     2571                                                                              10      6.0    16%    11%                                          11     2562                                                                              12      6.4    13%   9.0%                                          11     2570                                                                              12      7.4     9%   3.8%                                          11 fused                                                                             2633                                                                              12      6.4    10%   5.6%                                          11 fused                                                                             2634                                                                              12      6.7     8%   6.4%                                          12     2590                                                                              10      6.1    9.0%  9.0%                                          12     2602                                                                              12      4.3    9.5%  4.0%                                          __________________________________________________________________________

FIG. 12 illustrates a method for cutting the slots 22 in the injectorplate 16. The injector plate 16 is mounted by way of a chuck 30 on anX-Y table 31 driven by an X-Y motor 32, which is controlled by a controlcircuit or computer 33. The pattern of the outside periphery of the slotis programmed into the control circuit or computer 33 to drive the X-Ytable 31 in that designated pattern. During this movement, a laser beam34 from a laser 35 melts through the injector plate 16. After the entirepattern has been described by the laser beam, the interior portion ofthe pattern simply falls out to leave the desired slot in the injectorplate. We used an "Anorad III" X-Y table as the table 31 and motor 32,which is available from the Anorad Company of Hauppauge, N.Y. A "PhotonSources Model Number 108" carbon dioxide laser, commercially availablefrom the Photon Sources Company of Livonia, Mich., was used as laser 35and was operated in a continuously pulsed repetition mode. A pulselength of fifty milliseconds with an off time of ninety milliseconds wasused, resulting in a pulse period of 140 milliseconds and a frequency of7.14 Hertz. The laser current was adjusted to achieve forty wattsaverage power. The injector plate 16 was a glass quartz plate havingthicknesses between forty and eighty mils. The X-Y table 31 was drivenat a rate of 2.25 inches per minute. Air was directed at the impingementof beam 34 on the injection plate 16 at forty pounds per square inchdelivered through a 0.06 inch orifice at a distance of 0.07 inch abovethe plate. With these parameters, the laser cut through the plate as itwas moved by the X-Y table to generate a slot having the desireddimensions without significantly damaging the baffle plate 15.

The various embodiments shown and described are intended to be merelyillustrative of the inventive concept. The process is inherentlyempirical in that the slot widths are experimentally tailored tocompensate for differences in the epitaxial growth rate, which may varywith variations in composition, flow rate and other parameters. Byrotating the wafer as described, there are substantially no thicknessvariations at a common radial distance. Thickness differences atdifferent radial distances, that would occur with a uniform flow rate,can be compensated by variations in the width of a single slot orplurality of slots as described. While a vertical reactor is preferred,other configurations could be used. It is known that II-VI epitaxiallayers can be made by MOCVD, and the invention can be used in suchprocesses. Various other embodiments and modifications may be made bythose skilled in the art without departing from the spirit and scope ofthe invention.

We claim:
 1. A method for making a semiconductor device comprising thesteps of: forming a substrate of a monocrystalline semiconductorcompound selected from the group consisting of group III-V compounds andgroup II-VI compounds; heating the substrate; epitaxially growing alayer on a first surface of the substrate comprising the step ofexposing the heated substrate to a gas comprising a metalorganiccompound and a second compound; said metalorganic compound comprising anorganic component and a metal selected from the group consisting ofgroups III and II of the periodic table; the second compound comprisingan element selected from the group consisting of groups V and VI of theperiodic table; said exposure causing a reaction of the metalorganiccompound and the second compound which results in the epitaxial growthon a surface of the substrate of a semiconductor compound comprising ametal which originally constituted part of the metalorganic compound andan element which originally constituted part of the second compound,characterized by the steps of:providing an injection plate that issubstantially parallel to the first surface of the substrate; theinjection plate having first and second parallel surfaces and an outerperiphery, the second surface facing the first surface of the substrate;providing in the injection plate at least one slot communicating withthe first and second surfaces, the slot having a non-uniform width whichis narrower at a region relatively near the center of the plate than ata region relatively near the periphery of the injection plate; rotatingthe substrate during deposition; and directing the gas to flow over thefirst surface of the injection plate in a substantially radially inwarddirection from a peripheral part of the injection plate toward thecenter of the injection plate, whereby at least part of the gas istransmitted through said slot before reaching the first surface of thesubstrate.
 2. The method of claim 1 further characterized in that:atvarious locations along its length, the width of each slot variesapproximately inversely with the rate of epitaxial growth at acorresponding location of the substrate surface, whereby the rate of gasinjection through each slot compensates for differences in the rate ofepitaxial growth along the substrate surface.
 3. The method of claim 2further characterized in that:the gas is directed to flow over theinjection plate in a radially inward direction toward the center of theinjection plate; the slots are arranged radially with respect to thecenter of the injection plate; and each slot is narrower at the endnearest the center of the injection plate than at the end nearest theperiphery of the injection plate.
 4. The method of claim 3 furthercharacterized in that:the injection plate contains two slots arranged180 degrees apart with respect to the injection plate center.
 5. Themethod of claim 3 further characterized in that:the injection platecontains four slots arranged ninety degrees apart with respect to theinjection plate center.
 6. The method of claim 3 further characterizedin that:the slots are each made by directing at the injection plate alaser beam of sufficient power to melt locally through the injectionplate, and causing relative movement of the plate with respect to thelaser beam such that the point of impingement of the laser beam on theinjection plate describes a path corresponding to the periphery of theslot.
 7. The method of claim 3 further characterized in that:a baffleplate is located parallel to the injection plate and opposite thesubstrate; and the gas is directed against the center of the baffleplate and is guided around the periphery of the baffle plate, thereafterto follow a radially inward direction along the injection plate.
 8. Themethod of claim 3 further characterized in that:the semiconductorcompound is a group III-V compound, the metal is a group III metal andthe element is a group V element.
 9. The method of claim 8 furthercharacterized in that:the semiconductor compound is indium phosphide,the metal is indium and the element is phosphorous.
 10. A method formaking a semiconductor device comprising the steps of:heating a crystalsemiconductor substrate; rotating the substrate; locating a flatinjection plate generally parallel to an upper surface of the substrate;directing a gas comprising a metalorganic component and a secondcomponent over a surface of the injection plate opposite the substrate;said directing step comprising the step of causing the gas to flowradially inwardly toward the center of the injection plate; causing thegas to flow through the injection plate at a rate which is higher at aregion relatively far from the center of the injection plate than at aregion relatively near the center of the injection plate; themetalorganic and second components reacting with the substrate to forman epitaxial layer on the upper surface of the substrate.
 11. The methodof claim 10 wherein:the rate with respect to distance at which the gasis caused to flow through the injection plate varies with distance so asapproximately to compensate for variations with respect to distance inthe rate at which the epitaxial layer would form assuming a uniform gassupply.
 12. The method of claim 11 wherein:the substrate is asemiconductor compound selected from the group consisting of group III-Vcompounds and group II-VI compounds.
 13. The method of claim 12wherein:the metalorganic compound comprises an organic compound and ametal; and the second compound comprises an element which is identicalto an element of the substrate.
 14. The method of claim 13 wherein:thegas is caused to flow over the injection plate from the periphery of theinjection plate radially inwardly toward the center of the injectionplate; and slots in the injection plate cause the gas to flow throughthe injection plate predominantly at regions corresponding to peripheralregions of the wafer.